1. Field of the Invention
The present invention relates to a copier, printer, facsimile apparatus or similar image forming apparatus and a process cartridge for use in the same and more specifically to a tandem image forming apparatus using a simultaneous developing and cleaning system.
2. Description of the Background Art
An image forming apparatus of the type using an electrostatic image transfer system is conventional and configured to form an electric field between a photoconductive drum or similar image carrier and an intermediate image transfer body, sheet conveyor or similar moving member for thereby transferring a toner image formed on the image carrier. In this type of image forming apparatus, some toner is left on the image carrier after the transfer of the toner image to a subject body, e.g., the intermediate image transfer body or a sheet or recording medium. If part of the image carrier on which such residual toner is present is subject to the next image formation, then irregular charging or similar defective charging occurs on the above part of the image carrier and lowers image quality. It is a common practice to remove the residual toner from the image carrier with a cleaning device.
The problem with the cleaning device mentioned above is that it needs an extra space for accommodating a waste toner tank configured to store the residual toner collected from the image carrier and a recycling path along which the residual toner is conveyed to be reused, making the entire apparatus bulky. Particularly, a current trend in the imaging art is toward a tandem image forming apparatus that assigns a particular image carrier to each color in order to meet the increasing demand for high-speed color image formation. If the cleaning device is applied to this kind of image forming apparatus, then a particular cleaning device must be assigned to each of a plurality of image carriers, making the above problem more serious.
To solve the problem stated above, Japanese Patent No. 3,091,323, for example, discloses an image forming apparatus using a simultaneous developing and cleaning system that causes a developing device to collect the residual toner. More specifically, the developing device, originally expected to develop a latent image, is used as cleaning means at the same time, so that a particular cleaning device does not have to be assigned to each image carrier. This contributes a great deal to the size reduction of the apparatus.
Japanese Patent mentioned above further teaches a charging device for the above image forming apparatus that includes a charge roller held in contact with the image carrier for uniformly charging the image carrier. Conventional systems for uniformly charging an image carrier are generally classified into a contact or vicinity type of charging system using a charge roller or similar charging member contacting or adjoining the image carrier and a non-contact type of charging system using a corona charger or similar charger. The non-contact type of charging system has a problem that it produces ozone, NOx (nitrogen oxides) and other discharge products, which are undesirable from the environment standpoint. In this respect, the contact or vicinity type of charging system, which produces a minimum of discharge products, is superior to the contact or vicinity type of charging system. Presumably, therefore, the apparatus taught in the above document promotes both of the size reduction of the apparatus and the reduction of discharge products.
However, the apparatus, using the simultaneous developing and cleaning system and contact or vicinity type of charging system has the following problem left unsolved. Before the residual toner present on the image carrier is conveyed to a developing zone, it contacts and deposits on the charging member, obstructing uniform charging. This prevents the charging member from charging the surface of the image carrier to an expected potential or causes irregular charging or similar defective charging to occur, resulting in short image density, background contamination and other defects. This problem is not particular to the apparatus using the simultaneous developing and cleaning system, but arises so long as the residual toner is conveyed to a position where the image carrier and charging member contact each other without being removed from the image carrier.
On the other hand, a blade type of cleaning device configured to clean the surface of the image carrier with a cleaning blade is predominant today because it sufficiently reduces undesirable black stripes extending in an image in the direction of movement of the above surface. Stated another way, with a bladeless type of cleaning device that does not use a cleaning blade, it is difficult to sufficiently control such black stripes. The bladeless type of cleaning device may use a brush roller for collecting the residual toner or a bias applying member for electrostatically collecting the residual toner. The simultaneous developing and cleaning system stated earlier is one of the bladeless type of cleaning systems.
The blade type of cleaning system has a problem that the edge of the cleaning blade strongly rubs and therefore shaves the entire surface of the image carrier and thereby reduces the life of the image carrier. Particularly, to meet the increasing demand for the size reduction of an image forming apparatus, the circumferential length of the image carrier is decreasing. For example, the diameter of a photoconductive drum, which is a specific form of the image carrier, is decreasing. When the circumferential length of the image carrier is reduced, the cleaning blade is required to rub the image carrier a larger number of times for a single image. It follows that when the blade type of cleaning system is applied to such an image carrier, the life of the image carrier is critically reduced.
By contrast, the bladeless type of cleaning system, which rubs the image carrier more softly than the blade type of cleaning system, successfully extends the life of the image carrier. In addition, load exerted by the bladeless type of cleaning system on the image carrier is lighter than load exerted by the blade type of cleaning system, reducing drive load to act on a driveline assigned to the image carrier. The simultaneous developing and cleaning system, in particular, does not need the extra space to be assigned to the waste toner tank and recycle path stated previously. In this sense, among some different bladeless cleaning systems, the simultaneous developing and cleaning system is advantageous in that it reduces the overall size of the apparatus, while achieving the above advantages at the same time.
It is a common practice with the bladeless type of cleaning system to press a brush roller or similar brush member against the image carrier and cause the former to rub the latter. In the case of a brush roller, the tips of bristles are pressed against the image carrier for scraping off the residual toner. Further, in the simultaneous developing and cleaning system, after the brush member, pressed against the image carrier, has rubbed the image carrier for weakening the adhesion of the residual toner to the image carrier, the developing device collects the residual toner, so that the residual toner can be easily collected.
However, the conventional bladeless type of cleaning system has a drawback that the bristles of the brush member, pressed against the image carrier, collapse and lose the expected function due to aging. Particularly, the role of the brush member is significant in the bladeless type of cleaning system as to the removal of residual toner, compared to the blade type of cleaning system. Therefore, the malfunction of the brush member ascribable to collapse adversely influences image quality more in the bladeless type of cleaning system than in the blade type of cleaning system.
The conventional bladeless type of cleaning system has another problem to be described hereinafter. Silica, zinc stearate and other additives contained in toner grains sometimes part from the toner grains due to, e.g., mechanical stresses acting during image formation. If such additives parted from the toner grains are pressed against the image carrier by a developer in a developing zone or by the brush member over a long time, then the additives adhere to the image carrier in the form of a thin film. This phenomenon is generally referred to as filming. Filming weakens the adhesion of the toner grains to the image carrier and thereby blurs or otherwise disfigures an image. Because the bladeless type of cleaning system rubs the image carrier with a weaker force than the blade type of cleaning system, as stated earlier, it cannot sufficiently shave off the film.
Generally, an image forming apparatus is developed with priority given to either one of image quality, i.e., the obviation of black stripes and the extension of the life of the image carrier and size reduction of the apparatus in accordance with the design object and desired characteristics of the apparatus. However, when priority is given to image quality, the life of the image carrier is short and needs frequent replacement. This not only obstructs efficient maintenance, but also increases user's expense. On the other hand, when priority is given to the life of the image carrier and size reduction, it is difficult to sufficiently reduce black stripes and therefore to enhance image quality.
Technologies relating to the present invention are also disclosed in, e.g., Japanese Patent Laid-Open Publication Nos. 8-137198, 8-137205, 9-211979, 11-190931, 2000-194242, 2000-242152, 2001-75448, 2001-117317 and 2001-356614.